1. Field of the Invention
The present invention relates to electronic supervision and control equipment for a radar installation. More specifically, a transmit/receive module is provided which has operating modes specifically directed to several desirable features of a phased array radar installation. Among the operating modes available are, an antenna command mode, a receive mode, a transmit mode, a receive-calibrate mode, a BIT/FIT mode and a transmit-calibrate mode. Error correction is provided in both an open loop and a closed loop mode to compensate for deviations from desired performance.
2. Description of the Prior Art
In the use of radar antennae in systems of the type designed for precisely locating remote airborne objects, some of which may be taking active steps to avoid being detected, it is extremely important that the radar antenna perform exactly in accordance with the intended manner of operation. The control of the antenna and the manipulation of the signals transmitted and received by the antenna are performed in part by electronic, electrical or electromechanical equipment having very tight operating tolerances. It has been a continuing objective to improve the performance of antenna control equipment and the associated radar performance.
A component of advanced radar control equipment which provides control over the rapid scanning of the environment via an active aperture array type radar is commonly referred to as a transmit/receive module (T/R module). The use of advanced equipment employing T/R modules overcomes the severe technical problems which limit the performance of traditional mechanically scanned radar equipment. Mechanical scanning via antenna rotation is eliminated in favor of electronic phase shifting of signals transmitted (and received) by a stationary antenna array. By providing a separate T/R module associated with each element of the antenna array, extremely fast directional changes may be made in the beam direction without the need for any physical rotation of the antenna structure. This ultrafast beam-direction pointing is made possible through the precise electronic control of the phase of the signal at each antenna element.
The overall operation of an electronically scanned antenna involves the provision of a signal to be transmitted, splitting the signal into a plurality of signal paths, each path being associated with an individual element of the antenna array, and directing the signal, via these signal paths, to each element of the antenna array with the phase of the signal at each element modified as necessary to cause the transmitted signal to form a beam which is pointed in a desired direction. In the receive mode, the phased array antenna system is also directional, this time providing directional control by adjusting the phase of the signals received at each element of the array such that signals from the desired direction are additive and signals from other directions tend to cancel each other out when added together since they are not in proper phase relation. This is done by providing a phase shifter in the signal reception path from each element and then combining the "in phase" signals in a power combiner to obtain the signal received from the predetermined direction.
In addition to the improved control over beam-direction pointing, the T/R module may be employed at the receiver front end in conjunction with a low-noise amplifier (preamplifier) to improve the sensitivity of the radar by eliminating the transmission losses from the antenna to the preamplifiers of common radars where there is no T/R module. Another improvement offered is in system efficiency where the application of power gain at each element avoids losses associated with corporate-fed power splitting arrangements previously employed.
A phase shifting function can be performed by any number of techniques well known to those familiar with phased array radar systems. Examples of various phase shifting approaches are disclosed in U.S. Pat. No. 4,044,360 and in a textbook referenced therein entitled "RADAR HANDBOOK" by Merrill I. Skolnik (McGraw-Hill 1970). The use of phase shifters in radar applications to provide phased array radars is well known. An example of a phased array radar is shown in U.S. Pat. No. 3,990,077, "Electrically Scanned Antenna For Direction Error Measurement", filed in 1975.
The necessity for providing identical signals to each antenna element, although phase shifted with respect to each other, has presented a challenging phase shifting coordination problem. One type of arrangement previously employed is known as the space-feed phased array radar. In this approach, a signal may be radiated from a horn onto the back surface of a panel of the array. An array of openings in the panel are fitted with feed-through type elements which receive a portion of the radiated signal, provide a preprogrammed phase shift, and re-radiate the signal. The programming of the phase shift of each of the feed-through radiator elements is the control mechanism for beam pointing. This approach represented a significant improvement over the previously employed corporate-feed arrangements due to the complexity of the hardware needed to precisely provide the signal to each of the radiating elements U S. Pat. No. 4,044,360, mentioned above, explains the benefits of the space-feed relative to corporate feed arrangements.
An early disclosure of the use of phase shifters to provide control over beam pointing is U.S. Pat. No. 3,864,689 "Hybrid Scan Antenna", filed in 1973, where the use of multiple wave-guide antenna sections are made to simulate a single longer section. In 1978, application was made for a system to simplify the previously complex corporate-fed phased array antenna systems and the resulting U.S. Pat. No. 4,257,050 "Large Element Antenna Array With Grouped Overlapped Apertures" discloses a system where, for instance, 40 beamwidths of scanning may be accomplished through the use of only 8 phase shifters and 8 mini-position switches. A semiconductor based phase shifter suitable for operation at frequencies of several Ghz/s is disclosed in U.S. Pat. No. 4,450,372 "Electronic Control Variable Phase Shift Device Comprising A Long Gate Field-Effect Transistor And A Circuit Using Such A Device", filed in 1982. Still another approach to phase shifting is disclosed in U.S. Pat. No. 4,480,254 "Electronic Beam Steering Methods And Apparatus", filed in 1982 which discloses the use of dielectric prisms which provide phase shift in response to the electric field strength to which the prism is exposed.
U.S. Pat. No. 4,359,742 "Dual Switch Multimode Array Antenna", filed in 1980, discloses a radar antenna system operable in both a transmit and a receive mode. U.S. Pat. No. 4,376,281 "Multimode Array Antenna", filed in 1980, discloses another radar antenna system designed for both transmit and receive operation.
U.S Pat. No. 4,450,372, mentioned above, discloses a transmit/receive radar system which employs a separate phase shifter for each antenna element. T/R switches are shown with intervening transmit and receive channel circuitry. Within the T/R module for each antenna element there are phase shift elements provided in both the transmit channel and in the receive channel in order to provide antenna aiming, i.e., beam pointing. Separate control circuitry is employed to provide the control signals for directional control.
FIG. 1 shows a typical prior art antenna arrangement where terminal 101 receives a transmit drive signal for transmission, and receives the signals received by the antenna. Phase shifter 103 receives phase control signals via phase control lines 104a-d, and provides a properly phased signal in response to the receipt of either an incoming or outgoing signal. The phase control regulates the beam direction of the phased array antenna. T/R switch 105 selects either transmit or receive operation by connecting either the transmit branch 106 or the receive branch 108 between the phase shifter 103 and the antenna element 110. The power amp 107 amplifies signals to be transmitted when in the transmit mode, i.e., when the transmit branch is in the circuit, while preamplifier 109 amplifies received signals when the system operates in the receive mode, and the receive branch is in the circuit.
The arrangement of FIG. 1 is limited in overall performance due to the tight tolerances which must be met for performance improvements. For instance, when 60 dB sidelobe suppression is desired, 0.5 dB RMS amplitude and 5.degree. RMS phase tracking are required. If sidelobe suppression of 75 dB is required, the system requirements are tightened. This would be fine if such tight tolerances could be met. However, even the tolerances for 60 dB sidelobe performance is stressing the capabilities of existing technology. Additionally, the prior T/R arrangements are subject to temperature variations, power-supply variations, duty cycle and pulse-width changes and frequency changes in normal operation. These effects result in large phase and amplitude variations in each module, causing poor tracking and sidelobe performance.
A phase correction circuit for a phase shifter is disclosed in U.S. Pat. No. 4,649,553 "Microwave Digital Phase-Shifter Apparatus And Method For Construction". This microwave digital diode phase shifter provides improved phase error and an improved carrier and spurious sideband suppression by utilizing a digital correction scheme. It also provides reduced amplitude modulation error by employing a GaAs amplifier operating at saturation.